The primary objective of the study is to determine whether modafinil at 255, 340, 425, or 510 mg'day is more effective than a placebo in alleviating the symptoms of Attention-Deficit/Hyperactivity Disorder (ADHD) in adults as determined by the change from baseline in the total score from the investigator-completed Adult ADHD Investigator Symptoms Rating Scale (AISRS) the at endpoint (last post-baseline observation [week 9 or early termination]).